1. Field of the Invention
The present invention relates to transporting flat objects such as semiconductor wafers or masks between successive stages in a semiconductor fabrication process, the objects being transported in front-opening transport boxes, in particular in so-called front opening universal pods (FOUPs) that have a side opening, or in standard mechanical interface (SMIF) pods that have a bottom opening.
2. Description of the Prior Art
Wafers and masks are commonly transported in FOUP or SMIF pods in order to protect wafers and masks against the residual pollution that is present, even in “clean” rooms. The transport pod isolates the wafers and masks from the atmosphere that exists in a clean room.
In the processes that are presently the most commonly used, wafers and masks are transported in a FOUP type or SMIF type pod that can be opened after being coupled to an interface situated at the inlet to a piece of equipment. The transport pod remains at atmospheric pressure, and its internal atmosphere is air or nitrogen, likewise at atmospheric pressure.
An example of such a pod is described in the document US-2001/0041530 and EP-1 204 138. The transport pod comprises a leakproof peripheral wall with an inlet/outlet opening closable by a pod door having front sealing means and peripheral sealing means for keeping the zone for coupling with the interface isolated from the outside atmosphere.
The interface is a system enabling the transport pod to be positioned, the pod door to be opened, and a flat article to be held and transported so as to be transferred between the transport pod and the piece of equipment.
Usually, the equipment interface is itself provided with a robot for holding and moving flat articles between the transport pod and an evacuation chamber of the equipment. After passing into the interface, the flat article is transferred into the evacuation chamber of the equipment.
The evacuation chamber of the equipment is periodically put back to atmospheric pressure so as to be connected to the equipment interface for transferring flat articles. The evacuation chamber is likewise periodically evacuated so as to provide communication with a treatment chamber of the equipment for the purpose of transferring flat articles. These changes in gas pressure require both pumping operations and operations of returning to atmospheric pressure to be performed, and those operations take time and lead to gas flows. Such gas flows can move particles, thereby leading to particulate contamination of the flat articles. Furthermore, when the flat article is at atmospheric pressure, it can be subjected to contamination by molecules present in the atmosphere. The contamination may relate to the interface, the transport pod, or the flat articles.
In order to improve the isolation provided by the transport pod relative to the atmosphere, U.S. Pat. No. 5,382,127 has already proposed using transport pods in which the inside atmosphere is at a pressure that is higher than the outside atmosphere. That solution is not satisfactory, since for communication with a piece of equipment it remains necessary to apply pumping to the atmosphere into the transport pod, and that implies setting up a flow of gas which might lead to particulate contamination.